The present invention relates to a brake-force regulator and more particularly to a brake-force regulator for a dual-circuit brake system.
From German Patent DE-AS No. 2,265,106 a brake-force regulator is known in which the rear wheels of a vehicle are controlled by two separate brake circuits. The regulator includes two pressure-fluid valves arranged in tandem, each valve controlling the fluid connection between a fluid source and a wheel-brake cylinder of a circuit. The first valve is a pressure reducing valve including an axially slidable stepped piston having acting on it a control force introduced from without. The smaller area of the stepped piston is acted upon by the pressure of the fluid source, and its larger area is acted upon by the pressure supplied to the wheel brakes in opposition to the control force. The second valve is a pressure-limiting valve whose closure member is arranged in an axially movable second piston and bears against the regulator housing through a tappet. On the side of the second piston close to the stepped piston, the closure member of the first valve bears against the side of the second piston adjacent the stepped piston through a tappet.
If both brake circuits are operative, the known brake-force regulator functions as follows:
In the regulator's inactive position, both valves are open and, as a result, the fluid flows to the wheel cylinders unhindered at the beginning of braking. By increasing the pressure in the brake system, the stepped piston is displaced against the control force. Since at the same moment the pressure in the outlet chamber of the second brake circuit would preponderate, the second piston follows the stepped piston until the second valve closes. When the pressure continues to increase, the stepped piston continues moving against the control force so that the first valve closes, too. As a result, outlet chambers are pressurized to a reduced braking pressure. The second piston ensures that the pressures in both outlet chambers are of equal magnitude.
If the second brake circuit fails, the function of the pressure-reducing valve in the stepped piston will remain unchanged, only the change-over pressure of the valve will be lower. Should, however, the first brake circuit fail, with the pressure in the second brake circuit increasing the second piston will be displaced against the control force, causing the stepped piston to follow its movement, until the second valve closes. A further increase of the braking pressure beyond the changeover pressure is not possible because the second valve can only operate as a pressure limiter in the event of failure of the first circuit.
The change-over pressure of the pressure-limiting valve is attained at a very low pressure if the first brake circuit has failed. As a result, only a minor braking effect is achieved at the brake associated with the second brake circuit. If the second brake circuit fails, the first valve continues to function as a pressure-reducing valve. However, the change-over pressure and, consequently, the braking effect are lower. It is, however, in the event of a brake-circuit failure, important to achieve the best possible braking effect with the still intact brake circuit. Therefore, a reduction of the braking effect is far from being desirable in the event of a brake-circuit failure.